Abstract

The 1883 eruption of Krakatau volcano, Indonesia produced approximately 12.5 km3 (DRE) of magma, 90% rhyodacite, 4% mafic dacite, and 1% andesite (5% lithic material assumed). Melt compositions in the erupted products based on the study of glass inclusions and matrix glasses span a minimum range of 13 wt.% SiO2. Well defined linear trends of major elements have been successfully modelled by incremental fractional crystallization of plagioclase, pyroxene, and Fe-Ti oxides from andesitic parental magma to produce rhyodacite. This fractional crystallization model is supported by trace-element and isotopic data. The 1883 magma chamber was compositionally and thermally zoned with an upper portion of homogenous rhyodacite at a temperature of 880-890°C overlying more mafic dacite at 890-913°C, and andesite at 980-1000°C. Mafic dacite represents a hybrid magma formed by mixing small amounts of andesite with larger amounts of evolved rhyodacite. Evidence for magma mixing consists of banded pumices, glass inclusions that are more mafic than whole rock and matrix glass compositions, heterogeneous matrix glass compositions, and disequilibrium phenocrysts of anorthite-rich plagioclase in rhyodacite and mafic dacite. The 1883 rhyodacitic magma was probably not water saturated until it reached a shallow depth in the crust of 4-5 km. Glass inclusions indicate pre-eruption dissolved volatile content in rhyodacite and mafic dacite of 4.0 ± 0.5 wt.%. Estimated sulfur discharge from erupted magma is 2.8 _ 1012 g S, and estimated Cl discharge is 9.7 _ 1012 g Cl. Two potential sources of additional sulfur from this eruption may be vaporization of seawater during entrance of pyroclastic flows into the sea, and degassing of nonerupted andesitic parental magma lying beneath evolved rhyodacitic magma in the zoned 1883 chamber.